The present invention relates to methods and apparatus for testing a subterranean formation.
Formation testing operations are commonly conducted to determine the production potential of oil and gas wells. As is well known in the art, these tests are conducted using formation testing strings. A typical formation testing string will include a tester valve and a packer. The tester valve is positioned in the testing string above the packer and, typically, both the tester valve and the packer are positioned near the end of the testing string. When closed, the tester valve operates to block fluid flow through the interior of the testing string.
In conducting a formation test, the testing string is lowered in the well bore until the end of the string reaches the depth of the formation to be tested. The packer is then set in the well bore at a point above the formation. Once the packer is set and the testing string is in place, the formation and the interior of the testing string can be isolated from the well bore annulus. As used herein, the term well bore annulus refers to that portion of the well bore located above the packer and outside of the testing string.
With the formation isolated in the manner just described, formation parameters such as formation flow, pressure, and rapidity of pressure recovery can be determined by alternately opening the tester valve to allow formation flow and closing the tester valve to block formation flow. Pressure readings are taken throughout this procedure in order to determine the production capability of the formation. If desired, a fluid sample can be taken from the formation by including a sampling tool in the testing string.
The testing string also typically includes a circulation valve positioned above the tester valve. At the end of the formation testing program, the circulation valve is opened and formation fluid is circulated out of the testing string. The packer is then released and the testing string is withdrawn from the well bore.
As the testing string is being lowered to its final position in the well bore, drill stem pressure tests are commonly conducted in order to determine if the string contains any leaks. In conducting a drill stem pressure test, an upper interior portion of the testing string is taken out of fluid communication with the well bore. The pressure inside the upper portion of the string is then increased (e.g., by pumping into the testing string) and maintained in order to determine if any fluid escapes therefrom. If a leak is discovered, the portion of the testing string containing the leak must be withdrawn from the well bore so that the leak can be repaired. As is well known in the art, the cumulative length of testing string which must be withdrawn, for leak repair purposes, from the well bore and reinserted during the course of the string lowering process can be minimized by conducting frequent pressure tests as the string is lowered into the well bore.
Various types of tester valves and other downhole tools are known in the art. These include valves and tools which are operated by string rotation, string reciprocation, tubing pressure changes, or differential pressure changes. Annulus pressure operated tools are particularly well suited for offshore applications. Through the use of annulus pressure operated tools, testing string rotation and/or reciprocation is minimized so that the well's blowout preventers can be kept closed during most of the testing operation. By minimizing the amount of time which the blowout preventers must be kept open, annulus pressure operated tools operate to minimize safety and environmental hazards.
U.S. Pat. No. 4,633,952 discloses an annulus operated, multi-mode testing tool. The tool includes a drill pipe tester valve, a circulation valve, a nitrogen displacement valve, and/or a formation tester valve. U.S. Pat. No. 4,633,952 indicates than an independently actuated formation tester valve can be positioned in the testing string below the multi-mode testing tool.
U.S. Pat. No. 4,657,082 discloses a circulation valve which is actuated by changes in the pressure differential existing between the interior of the testing string and the exterior of the testing string. U.S. Pat. No. 4,657,082 indicates that the internal-external differential pressure operated circulation valve disclosed therein can be used in conjunction with a conventional rotation and/or reciprocation actuated circulating valve and an annulus pressure operated tester valve.
U.S. Pat. No. 4,655,288 discloses a tester valve which utilizes a lost-motion valve actuator. The tester valve of U.S. Pat. No. 4,655,288 is annulus pressure actuated. U.S. Pat. No. 4,655,288 also indicates that the tester valve disclosed therein can be used in conjunction with an annulus pressure operated circulation valve.